High-Power Mode-Locked Laser System and Methods of Use

ABSTRACT

A high-power mode-locked laser system is disclosed herein which includes at least one pump source, at least one laser cavity formed by at least one high reflector and at least one output coupler, and at least one ytterbium-doped optical crystal positioned within the laser cavity in communication with the pump source, the ytterbium-doped optical crystal configured to output at least one output signal of at least 20 W, having a pulse width of 200 fs or less, and a repetition rate of at least 40 MHz.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is continuation application of U.S. patentapplication Ser. No. 15/829,875 filed Dec. 2, 2017, entitled “High-PowerMode-Locked Laser System and Methods of Use,” which claims priority toU.S. Provisional Patent Appl. Ser. No. 62/429,830, filed on Dec. 4,2016, entitled “High-Power Ytterbium Doped Calcium Fluoride Mode-LockedLaser and Methods of Use,” the entire contents both of which areincorporated by reference herein.

BACKGROUND

High-power mode-locked laser systems are presently used in a variety ofapplications, such as multi-photon microscopy and device manufacturing.Presently, three types of high power, mode-locked laser systems arecommonly available for these applications: thin disk laser systems,chirped pulse fiber amplifier systems and bulk laser systems. Thin disklaser systems are a diode pumped solid state laser system which includesa thin layer of active gain material positioned on a heat sink. A pumpsignal from a diode pump source is incident multiple times on the activegain material, which produces an output signal in response thereto.Historically, disk laser systems have been capable of producing highaverage powers. However, disk laser systems have been largely incapableof reliably producing output signals having pulse widths of less thanabout 500 femtoseconds (hereinafter “fs”) and at high average power andhigh repetition rates. Further, disk laser systems require a complex andexpensive optical pumping configuration and thermal management system.Due to peak power limitations, fiber based high power, mode-lockedlasers require an oscillator and a chirped pulse amplifier whichincludes stretching the pulse prior to amplification and then subsequentcompression after amplification, thus adding cost and complexity to thesystem.

In contrast, bulk high power, mode-locked laser systems use opticalcrystals, such as Yb:YAG, Yb:CALGO, Yb:KYW or Yb:KGW, as the gainmaterial. While prior art bulk high power mode-locked laser systems haveproven useful in the past, a number of shortcomings have beenidentified. Often, the high power optical pumping of the optical crystalresults in one or more undesirable thermal effects within the opticalcrystal. For example, one or more thermal lenses may be created withinthe optical crystal, thereby reducing the output power of the lasersystem. Typically, the average output power of these prior art bulklaser systems is less than about 15 W. FIG. 1 shows graphically therange wherein a continuous wave mode-locked (CW-ML) signal is outputtedfrom the laser within prior art laser cavities as a function of averageoutput power versus average pump power from the pump source. As shown,the narrow CW-ML regime is terminated by the undesirable instabilityregime. As such, operations or systems that require a CW-ML signal arerestricted to relatively low optical average power applications. Inaddition, presently available bulk high-power mode-locked laser systemstend to be complex systems requiring multiple pump sources, complexthermal management systems, and the like.

Thus in light of the foregoing, there is an ongoing need for a simple,low cost high-power mode-locked laser system capable of producing shortpulses at high average powers. There is a further need for a simple, lowcost high-power mode-locked laser system capable of producing sub 200 fspulse durations with average powers of more than 20 W. Further, there isan ongoing need for a simple, low cost high-power mode-locked lasersystem capable of producing these short pulse durations and high averagepowers at sufficient repetition rates for applications. Further, thereis a need for a simple, low cost high-power mode-locked laser systemwith an extended CW-ML range for ease of manufacturing and robustness.

SUMMARY

The present application discloses various embodiments of a high-powermode-locked laser system and methods of use. In one embodiment, thepresent application discloses a high-power mode-locked laser systemwhich includes at least one ytterbium-doped optical crystal therein. Thehigh-power mode-locked laser system may include at least one pump sourceconfigured to provide at least one pump signal. The pump signal may bedirected into at least one laser cavity formed by at least one highreflector and at least one output coupler. Further, at least oneytterbium-doped optical crystal may be positioned within the at leastone laser cavity. The laser system having at least one ytterbium-dopedoptical crystal therein may be in communication with and pumped by thepump signal from the pump source and may be configured to output atleast one output at least one output signal having a output power of 20W or more and pulse width of about 200 fs of less.

In another embodiment, the present application is directed to ahigh-power bulk laser system. Like the previous embodiment, thehigh-power bulk laser includes at least one pump source. At least onelaser cavity formed from at least one high reflector and at least oneoutput coupler may be configured to receive the pump signal from thepump source. At least one bulk ytterbium-doped optical crystal may bepositioned within the laser cavity and is in communication with pumpsource. The bulk ytterbium-doped optical crystal may be configured tooutput at least one output signal 20 W and 200 fs which may be outputtedfrom the output coupler.

In yet another embodiment, the present application discloses ahigh-power laser. The high-power laser includes at least one pumpsource. At least one laser cavity formed by at least one high reflectorat least one output coupler may be configured to receive the pumpsignal. At least one gain media may be positioned within the lasercavity and may be in communication with the pump source. The gain mediamay be configured to output at least one output signal of at least 20 W,having a pulse width of 200 fs or less, and a repetition rate of atleast 40 MHz from the at least one output coupler.

Other features and advantages of the high-power mode locked laser systemand method of use as described herein will become more apparent from aconsideration of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel aspects of the high-power mode locked laser system and methodof use as disclosed herein will be more apparent by review of thefollowing figures, wherein:

FIG. 1 shows graphically the mode-locking regime of a prior arthigh-power laser system;

FIG. 2 shows a schematic of an embodiment of a high-power mode-lockedlaser system having a single diode pump source delivering a pump signalto the laser cavity via a fiber optic device;

FIG. 3 shows a schematic of another embodiment of a high-powermode-locked laser system having a single diode pump source deliveringmultiple pump signals to the laser cavity;

FIG. 4 shows a schematic of an embodiment of a high-power mode-lockedlaser system having a multiple diode pump sources delivering multiplepump signals to the laser cavity;

FIG. 5 shows a table comparing the performance of the novel bulk Yb:CaF₂laser systems described herein relative to the performance of prior artlaser; and

FIG. 6 shows graphically the improved performance and increasedmode-locking regime achieved by the embodiments of the Yb:CaF₂mode-locked laser system disclosed herein.

DETAILED DESCRIPTION

The present application is directed to various embodiments of ahigh-power ytterbium-doped calcium fluoride (hereinafter “Yb:CaF₂”)mode-locked laser system for use with various optical systems. Thevarious embodiments of the high-power Yb:CaF₂ mode-locked laser systeminclude a novel cavity design configured to take advantage of the uniquephotothermal properties of Yb-doped optical crystals such as Yb:CaF₂ toprovide output signals having a continuous mode-locking range spanningapproximately fifty percent (50%) of the upper region of the dynamicrange of the output power of the laser system while reducing oreliminating spectral instabilities at the upper regions of themode-locking window while having the lowest or nearly lowest ordertransverse spatial Gaussian beam (TEM₀₀) being resonant. In oneembodiment, the high-power Yb:CaF₂ mode-locked laser system describedherein may be configured to deliver nearly transform-limited, sub-300 fspulses with average output powers exceeding about 20 W, at repetitionrates greater than about 50 MHz. For example, in one embodiment, thebulk Yb:CaF₂ laser systems described herein may be configured to delivernearly transform-limited, sub-200 fs pulses with average output powersexceeding about 25 W, at repetition rates greater than about 70 MHz. Inanother embodiment, the bulk Yb:CaF₂ laser systems described in hereinmay be configured to deliver nearly transform-limited, sub-150 fs pulseswith average output powers exceeding about 30 W or more, at repetitionrates greater than about 80 MHz. The high-power Yb:CaF₂ mode-lockedlaser systems disclosed herein may be used in conjunction with harmoniccrystals, optical parametric oscillators, and similar devices forvarious multi-photon microscopy applications. Optionally, the high powerYb:CaF₂ mode-locked laser systems disclosed herein may be configured foruse in laser-based spectroscopy applications, including, for example,dual-comb spectroscopy applications. In another embodiment, thehigh-power Yb:CaF₂ mode-locked laser systems disclosed herein mayinclude at least one amplifier module positioned therein or coupledthereto, thereby providing a high-power mode-locked laser system.Optionally, the high-power Yb:CaF₂ mode-locked laser systems may be usedas a pump source for pumping near-infrared optical parametricoscillators (hereinafter “OPOs”) and/or mid-infrared OPOs, and/orfar-infrared OPOs, and/or optical parametric generators. In anotherapplication, the high-power Yb:CaF₂ mode-locked laser system describedherein may be used for manufacturing a variety of devices, such aswriting waveguides and the like. In yet another application, thehigh-power Yb:CaF₂ mode-locked laser system described herein may be usedin conjunction with non-linear optical materials, including fiber-basednon-linear optical devices and/or bulk nonlinear devices such assapphire, YAG or diamond to produce supercontinuum generation systemsand devices. In yet another application, the high-power Yb:CaF₂mode-locked laser system described herein may be used for producing adual comb source for spectroscopy applications. FIG. 2 shows a schematicdiagram of an embodiment of a high-power Yb:CaF₂ mode-locked lasersystem (hereinafter “laser system”). As shown, the laser system 200includes at least one pump source 202 configured to output at least onepump signal 204. In one embodiment, the pump signal 204 has a wavelengthof about 850 nm to about 995 nm. For example, in one embodiment, thepump signal 204 has a wavelength of about 979 nm. In another embodiment,the pump signal 204 has a wavelength of about 940 nm. Optionally, thepump signal 204 may have a wavelength of about 976 nm. In yet anotherembodiment, the pump signal 204 has a wavelength of about 917 nm. Inanother embodiment, the pump signal 204 may include multiple wavelengthstherein. Further, the pump source 202 may be configured to output acontinuous wave pump signal 204. In another embodiment, the pump source202 may be configured to output at least one pulsed pump signal 204.

In the illustrated embodiment, the pump source 202 comprises a singlediode-based pump source having multiple emitters therein, the emitterseach configured to output an optical signal. For example, in oneembodiment, single-diode pump source comprises multiple laser diodeemitters located within a single diode package or device, wherein eachemitter is configured to output an optical signal into a single fiberoptic device. As such, the single fiber optic device may be configuredto receive the optical signals from the multiple laser diode emittersand output a single pump signal. Optionally, multiple diode-based pumpsources may be used in the present system. Further, any variety ofalternate pump sources may be used with the laser system 200. Forexample, fiber lasers may be used as the pump source 202. As shown, atleast one fiber optic device 206 may be coupled to or otherwise inoptical communication with the single diode-based pump source 202 andconfigured to receive the multiple optical signals from the multipleemitters within the pump source 202 and output a single pump signal 204therefrom. In one embodiment, the fiber optic device 206 is configuredto output a single mode-mixed pump signal 204. Optionally, the fiberoptic device 206 may be configured to output a non-mode-mixed pumpsignal 204. Further, the fiber optic device 206 may be configured toalter the polarization of at least one pump signal from at least one ofthe emitters within the pump source 202. For example, the fiber opticdevice 206 may be configured to output a pump signal 204 having a singlelinear polarization. In the alternative, the fiber optic device 206 maybe configured to output a pump signal 204 having ellipticalpolarization. In another alternative, the fiber optic device 206 may beconfigured to output a depolarized pump signal 204. Optionally, thefiber optic device 206 may be configured to mix the modes, polarization,intensity, and the like of at least two optical signals received from atleast two emitters positioned within the pump source 202. Those skilledin the art will appreciate that the fiber optic device 206 may bemanufactured in any variety of lengths, transverse dimensions, and thelike. Further, in one embodiment, the fiber optic device 206 maycomprise multimode fiber with a core size of 100 microns, 200 microns orthe like, single-mode fiber, graded-index fiber, holey fiber, photoniccrystal fiber, and the like.

Referring again to FIG. 2, the pump signal 204 may be directed into atleast one optical system 210. In the illustrated embodiment, the opticalsystem 210 includes a first lens 212 and an optional second lens 214. Assuch, FIG. 2 shows a laser system 200 having a telescope of lens devuceor system configured to focus the telescope pump signal 204 into thelaser. Those skilled in the art will appreciate that any number or typeof optical components or device may be used in the optical system 210.For example, as shown in FIG. 2, at least one optional optical component216 may be positioned within or proximate to the optical system 210.Exemplary optional optical elements 216 include, without limitations,lenses, gratings, spectral filters, beam splitters, sensors, spatialapertures, shutters, modulators, attenuators, homogenizers, polarizers,and the like.

As shown in FIG. 2, the pump signal 204 may traverse through at leastone of the high reflector 220 and/or output coupler 260 and may beincident on at least one optical crystal system 230 positioned withinthe laser cavity 290. In the illustrated embodiment, the high reflector220 includes at least one optical coating configured to reflectsubstantially all (i.e., greater than about 99.9%) intra-cavity signal236 (i.e., light having a wavelength of about 1000 nm to about 1700 nm)while transmitting substantially all the pump signal 204 (i.e., lighthaving a wavelength of about 850 nm to about 995 nm) there through. Inthe illustrated embodiment, the high reflector 220 comprises at leastone planar body. In another embodiment, the high reflector 220 comprisesa curved or arcuate body. Further, the high reflector 220 may be coupledto at least one optical stage or mount. Optionally, the optical stagesupporting the high reflector 220 may comprise an adjustable mirrormount. In the alternative, the optical stage supporting the highreflector 220 may comprise a non-adjustable mirror mount.

Referring again to FIG. 2, at least one optical crystal system 230 maybe positioned within the laser cavity 290 and be configured to receiveat least a portion of the pump signal 204 therein. In one embodiment,the optical crystal system 230 comprises at least one optical crystal232 positioned on at least one crystal mount 234. In one embodiment, theoptical crystal 232 comprises at least one bulk, optical material.Exemplary bulk, optical materials include, without limitations,ytterbium doped calcium fluoride (hereinafter “Yb:CaF₂”) single crystalmaterials, Yb:CaF₂ ceramic materials, Yb:CALGO, other ytterbium dopedgain media and the like. Optionally, the Yb:CaF₂ material describedherein may include one or more additional dopants known in the art. Forexample, in one embodiment, the Yb:CaF₂ materials described herein mayhave an absorption spectrum which extends from about 880 nm to about1100 nm. Further, the Yb:CaF₂ materials may be configured to generate atleast one intra-cavity signal 236 having a wavelength from about 1000 nmto about 1700 nm or more in response to being pumped by the pump signal204. Optionally, multiple optical crystals 232 may be positioned withinlaser cavity 290. The optical crystal 232 may include at least one facetorthogonal to the incident pump signal 204 or at least one facet whichis angled relative to the incident pump signal 204. Further, the pumpsignal 204 may propagate along any direction of the optical crystal 232.As such, the optical crystal 232 may comprise a <100>-cut crystal. Inanother embodiment, the optical crystal 232 may comprise a <110>-cutcrystal. Optionally, the optical crystal 232 may comprise a <111>-cutcrystal. Optionally the crystal may be cut in any other propagationdirection.

As shown in FIG. 2, the optical crystal 232 may be positioned on atleast one crystal mount 234 configured to securely and preciselyposition the optical crystal 232 within the laser cavity 290. In theillustrative embodiment, the laser cavity 290 comprises a linear cavity.Optionally, the laser cavity 290 may comprise a folded cavity, aZ-cavity, a ring cavity, and the like. As such, the laser system 200 mayinclude one or more additional fixed and/or adjustable fold mirrors,planar mirrors, curved mirrors, dichroic mirrors, dispersion-managementmirrors, and the like configured to permit the user to configure thatcavity architecture as desired. In the illustrate embodiment, thecrystal mount 234 may be in communication with at least one thermalcontrol system 238. For example, if present, the thermal control system238 may include one or more thermoelectric chillers, fluid sources,heaters, thermocouples, sensors, and the like. During use, the thermalcontrol system 238 may be configured to monitor and control thetemperature of the crystal mount 234 and optical crystal 232 positionedon the crystal mount 234. As such, crystal mount 234 may be manufacturedfrom any variety of materials, including material having a highcoefficient of thermal conductivity. Optionally, thermal control system238 may comprise one or more fans or similar devices configured to coolthe optical crystal 232 positioned on the crystal mount 234 throughconvection, thereby eliminating the need for the complex water-basedconductive thermal control systems used in many lasers presently. Assuch, the crystal mount 234 may include one or more features orelements, such as fins, heat sinks, and the like configured to aid theconvection cooling of the optical crystal 232. Further, the thermalcontrol system 238 may be in communication with at least one internal orexternal processor configured to monitor and maintain the opticalcrystal 232 at a desired temperature. Optionally, the laser system 200may be operated without a thermal control system 238.

Referring again to FIG. 2, the optical crystal 232 is configured togenerate at least one intra-cavity signal 236 in response to beingpumped by the pump signal 204. The intra-cavity signal 236 may bedirected into at least one mode-locking system 240 positioned within orproximate to the laser cavity 290. In one embodiment, the mode-lockingsystem 240 comprises at least one Kerr lens mode-locking system(hereinafter “KLM system”). In another embodiment, the mode-lockingsystem 240 comprises at least one saturable absorber, semiconductorsaturable absorber mirror (hereinafter “SESAM”), and/or hybrid KLM/SESAMsystem configured to permit self-starting mode-locking. In anotherembodiment, the mode-locking system 240 comprises a non-linear-opticalcrystal in which intentionally phase mismatched harmonic generationprovides the self-starting mode-locking. Those skilled in the art willappreciate that any variety of alternate mode-locking system and devicesmay be used with the laser system 200.

As shown in FIG. 2, at least one intra-cavity optical component 250 maybe positioned within the laser cavity 290. In the illustratedembodiment, two intra-cavity optical components 250 are positionedwithin the laser cavity 290, although those skilled in the art willappreciate that any number of devices may be used in various locationswithin the laser cavity 290. In one embodiment, the intra-cavity opticalcomponent 250 comprises a spatial filter. In another embodiment, theintra-cavity optical component 250 comprises a polarizer. For example,the laser system 200 may be operated in a linear polarization state,wherein the intra-cavity optical component 250 comprises an intra-cavitypolarization selection element. As such, the laser system 200 maysimultaneously operate in two orthogonal linearly polarized states in acollinear configuration. Additionally, the two polarization states mayoperate at slightly different repetition rates while occupying the samecavity volume reducing common-mode noise levels on the output signal280.

Optionally, any variety of optical components may be used as theintra-cavity optical component 250, including, without limitations,lenses, beam splitters, mirrors, optical filters, apertures, stops,irises, sensors, prisms, dispersion compensation devices or systems,group delay dispersion devices and systems, Gires-Tournoisinterferometer mirrors, modulators, optical flats, Brewster windows, andthe like. In another embodiment, the intra-cavity optical component 250comprises at least one harmonic generation device or crystal. Forexample, the intra-cavity optical component 250 may comprise at leastone harmonic generation device configured to produce at least one secondharmonic signal, third harmonic signal, fourth harmonic signal, and orthe like when pumped with at least one of the pump signal 204 and/orintra-cavity signal 236.

Referring again to FIG. 2, the laser system 200 includes at least oneoutput coupler 260, which, in combination with the high reflector 220,defines the laser cavity 290. In one embodiment, the output coupler 260is configured to transmit between about five percent (5%) to about fiftypercent (50%) of the intra-cavity signal 236, thereby permitting atleast a portion of the intra-cavity signal 236 to exit the laser cavity290, thereby producing an output signal 280 having a wavelength of about1000 nm to about 1700 nm. For example, in one embodiment, the outputsignal 280 has a wavelength between about 1000 nm and 1100 nm. Inanother embodiment the output coupler 260 is configured to transmit morethe about five percent (5%) and less than about fifty percent (50%) ofthe intra-cavity signal 236. Further, the unique bulk Yb:CaF₂ lasersystem 200 described herein (as well as the laser systems describedbelow and shown in FIGS. 3 and 4) may be configured to deliver nearlytransform-limited, sub-300 fs pulses with average output powersexceeding about 20 W, at repetition rates greater than about 50 MHz. Forexample, in one embodiment, the bulk Yb:CaF₂ laser systems described inherein may be configured to deliver nearly transform-limited, sub-200 fspulses with average output powers exceeding about 25 W, at repetitionrates greater than about 70 MHz. Optionally, there are applicationswhere the input peak power of the output signal 280 is limited, forexample, continuum generation in fibers, where self-focusing leads todamage of the fiber at less than 1 MW peak power and higher. For suchapplications the average power of the laser system 200 may be increasedby increasing the repetition rate of the laser system 200 while keepingboth the pulse energy and the pulse duration (and thus the peak power)fixed. Thus for a 100 nJ pulse with 100 fs duration, a repetition rateof 100 MHz would lead to a 10 W average power laser while a repetitionrate of 200 MHz would produce 20 W of average power, both with 1 MW ofpeak power. Such a higher repetition rate laser would produce a higheraverage power continuum source and be physically shorter and thus morecompact.

In another embodiment, the bulk Yb:CaF₂ laser systems described inherein may be configured to deliver nearly transform-limited, sub-150 fspulses with average output powers exceeding about 30 W or more, atrepetition rates greater than about 80 MHz. In another embodiment, thevarious laser systems disclosed herein may be configured to output atleast one output signal of at least 20 W, having a pulse width of 200 fsor less, and a repetition rate of at least 300 MHz. Optionally, thevarious laser systems disclosed herein may be configured to output atleast one output signal of at least 20 W, having a pulse width of 200 fsor less, and a repetition rate of at least 400 MHz. Further, as shown inFIG. 2, an intra-cavity optical component 250 may be a mirror configuredto reflect at least a portion of the pump signal 204 back into theoptical crystal 232. Optionally, the output coupler 260 may include anyadditional or alternate optical coatings applied thereto. Exemplaryadditional coatings may include, without limitations, polarizingcoatings, bandpass filter coatings, notch filter coatings, wavelengthselective coatings, and the like.

FIG. 3 shows a schematic of another embodiment of a high-power ytterbiumdoped calcium fluoride mode-locked laser system (hereinafter “lasersystem”). Like the previous embodiment, the laser system 300 includes asingle diode-based pump source 302 configured to output at least onepump signal 304. Like the previous embodiment, the laser system includesat least one fiber optic device 306 coupled to or otherwise incommunication with the single diode-based pump source 302. The pumpsignal 304 may have a wavelength of about 850 nm to about 995 nm. Forexample, in one embodiment, the pump signal 304 has a wavelength ofabout 979 nm. In another embodiment, the pump signal 304 has awavelength of about 940 nm. In another embodiment, the pump signal 304has a wavelength of about 976 nm. In yet another embodiment, the pumpsignal 304 has a wavelength of about 917 nm. In another embodiment, thepump signal 304 may include multiple wavelengths therein. Further, thesingle diode-based pump source 302 may be configured to output acontinuous wave pump signal 304. In another embodiment, the singlediode-based pump source 302 may be configured to output at least onepulsed pump signal 304.

Referring again to FIG. 3, the pump signal 304 outputted by the fiberoptic device 306 coupled to the single diode-based pump source 302 maybe directed into at least one optical system 310 as described above. Forexample, the optical system 310 may comprise at least one telescope,collimator, and the like. In the illustrated embodiment, the opticalsystem 310 includes a beam splitter configured to receive the pumpsignal 304 and form a first pump beam 308 a and at least a second pumpbeam 308 b. Optionally, the optical system 310 may include various fiberoptics devices, waveguides, lenses, mirrors, and the like permitting atleast one of the first and second pump beams 308 a, 308 b to be insertedinto the laser cavity 390 at any desired location.

As shown in FIG. 3, the laser cavity 390 may be defined by a highreflector 320 and an output coupler 360. Those skilled in the art willappreciate the laser cavity 390 maybe formed in any variety ofconfigurations, including, without limitations, linear configurations,folded cavities, Z cavities, ring cavities, and the like. As such, anynumber of planar or curved fold mirrors, reflectors, optical mounts, andthe like may be used to form any desired cavity architecture. As shown,the high reflector 320 is configured to permit at least a portion of thefirst pump beam 308 a to traverse there through. As such, like theprevious embodiment, the high reflector 320 may include one or moreoptical coatings applied thereto. The first pump beam 308 a may traversethrough the high reflector 320 and may be incident on at least oneoptical crystal 330 and/or optical crystal 332 positioned within thelaser cavity 390. In the illustrated embodiment, the high reflector 320includes at least one optical coating configured to reflectsubstantially all (i.e., greater than about 99.9%) of the intra-cavitysignal 336 (i.e., light having a wavelength of about 1000 nm to about1700 nm) while transmitting substantially all the first pump beam 308 a(i.e., light having a wavelength of about 850 nm to about 995 nm) therethrough. In the illustrated embodiment, the high reflector 320 comprisesat least one planar body. In another embodiment, the high reflector 320comprises a curved or arcuate body. Further, the high reflector 320 maybe coupled to at least one optical stage or mount. Optionally, theoptical stage supporting the high reflector 320 may comprise anadjustable mirror mount. In the alternative, optical stage supportingthe high reflector 320 may comprise a non-adjustable mirror mount.

Like the first pump beam 308 a, the second pump beam 308 b may bedirected into the laser cavity 390 and made to be incident upon theoptical crystal system 330 positioned within the laser cavity 390. Inthe illustrated embodiment, the second pump beam 308 b is directedthrough the output coupler 360. Optionally, the second pump beam 308 bmay be directed through to the output coupler 360 using one or moreoptical fibers, waveguides, mirrors, free space propagation systems, andthe like. For example, one or more fiber optic conduits may beconfigured to receive the second pump beam 308 b from the optical system310 and direct the second pump beam 308 b into the laser cavity 390 viathe output coupler 360. In another embodiment, one or more mirrors maybe configured to receive the second pump beam 308 b from the opticalsystem 310 and direct at least a portion of the second pump beam 308 binto the laser cavity 390.

Referring again to FIG. 3, at least one optical crystal system 330 maybe positioned within the laser cavity 390 and be configured to receiveat least a portion of the first pump beam 308 a and/or second pump beam308 b therein. Like the previous embodiment, the optical crystal system330 may comprise at least one optical crystal 332 positioned on at leastone crystal mount 334. Optionally, the optical crystal may comprise atleast one bulk, optical material. Exemplary bulk, optical materialsinclude, without limitations, ytterbium doped calcium fluoride “Yb:CaF₂”single crystal materials, Yb:CaF₂ ceramic materials, Yb:CALGO, Yb:KGW,Yb:KYW, Yb:glass, Yb:LuO, Yb:YCOB, Yb:LuScO, other Yb doped gain mediaand the like. Optionally, the Yb:CaF₂ material described herein mayinclude one or more additional dopants known in the art. For example, inone embodiment, the Yb:CaF₂ materials described herein may have anabsorption spectrum which extends from about 880 nm to about 1100 nm.Further, the Yb:CaF₂ materials may be configured to generate at leastone intra-cavity signal 336 having a wavelength from about 1000 nm toabout 1700 nm or more in response to being pumped by at least one of thefirst and second pump beams 308 a, 308 b. Optionally, multiple opticalcrystals 332 may be positioned within laser cavity 390.

As shown in FIG. 3, the optical crystal 332 may be positioned on atleast one crystal mount 334 configured to securely and preciselyposition the optical crystal 332 within the laser cavity 390. In oneembodiment, the optical crystal system 330 may be in communication withone or more thermal control systems (not shown) similar to the thermalcontrol systems described above (See Paragraph [0008], FIG. 2). Forexample, if present, the thermal control system may include one or morethermoelectric coolers, chillers, fluid sources, heaters, thermocouples,sensors, and the like. For cost and simplicity, often it is desirable toeliminate water cooling of the optical crystal 332 and thus eliminatethe components used in such a thermal control system, such as thechiller. In the illustrated embodiment, the optical crystal system 330comprises an air-cooled crystal mount system 334. Those skilled in theart will appreciate that the various laser systems described in thepresent application may operate well above known damage thresholdswithout substantial degradation of average power, even at elevatedtemperatures. Further, the cavity mode (the laser mode of theintra-cavity signal 336) is relatively insensitive to temperature due tothe weak thermal lens generated in the Yb:CaF₂ optical crystal duringuse as compared to the strong thermal lens created in other activeoptical materials. Thus, the laser systems disclosed in the presentapplication may be operated at temperatures of 30° C. or higher withouta substantial decrease in performance. As such, the crystal mount 334may include one or more features, fins, elements, and the likeconfigured to enhance convection cooling of the crystal mount 334, andby extension, cooling of the optical crystal 332.

Referring again to FIG. 3, the optical crystal 332 is configured togenerate at least one intra-cavity signal 336 in response to beingpumped by the first and second pump beams 308 a, 308 b. The intra-cavitysignal 336 may be directed into at least one mode-locking system 340positioned within or proximate to the laser cavity 390. The intra-cavityoptical component 350 may comprise a spatial filter. In anotherembodiment, the intra-cavity optical component 350 comprises apolarizer. Optionally, any variety of optical components may be used asthe intra-cavity optical component 350, including, without limitations,lenses, beam splitters, mirrors, optical filters, aperture, stops,irises, sensors, prisms, dispersion compensation devices or systems,group delay dispersion devices and systems, Gires-Tournoisinterferometer mirrors, modulators, optical flats, Brewster windows, andthe like. In another embodiment, the intra-cavity optical component 350comprises at least one harmonic generation device or crystal. Forexample, the intra-cavity optical component 350 may comprise at leastone harmonic generation device configured to produce at least one secondharmonic signal, third harmonic signal, fourth harmonic signal, and thelike when pumped with at least one of the first pump beam 308 a, secondpump beam 308 b, and/or intra-cavity signal 336. Optionally, the lasersystem may be cavity dumped to increase the pulse energy.

Referring again to FIG. 3, the laser system 300 includes at least oneoutput coupler 360, which, as stated above, in combination with the highreflector 320, defines the laser cavity 390. In one embodiment, theoutput coupler 360 is configured to reflect about five percent (5%) toabout fifty percent (50%) of the intra-cavity signal 336, therebypermitting at least a portion of the intra-cavity signal 336 to exit thelaser cavity 390, thereby producing an output signal 380 having awavelength of about 1000 nm to about 1700 nm. For example, in oneembodiment, the output signal 380 has a wavelength between about 1000 nmand 1100 nm. Optionally, the output coupler 360 may include anyadditional or alternate optical coatings applied thereto. Exemplaryadditional coatings may include, without limitations, polarizingcoatings, bandpass filter coatings, notch filter coatings,wavelength-selective coatings, and the like.

As shown in FIG. 3, at least one optical system or device 392 may becoupled to or positioned in optical communication with the laser cavity390 and configured to receive the output signal 380 and direct, modify,measure, or otherwise condition the output signal 380 prior to use. Forexample, in one embodiment, the external optical system 392 isconfigured to output at least one modified output signal 394.Optionally, the external optical system 392 comprises at least oneharmonic generation system configured to output one or more harmonicoptical signals in response to being irradiated by the output signal380. For example, the external optical system 392 may include at leastone second harmonic generation device therein. In another embodiment,the external optical system 392 may include at least one third harmonicgeneration device therein. In another embodiment, the external opticalsystem 392 comprises at least one frequency doubling device and at leastone optical parametric oscillator therein. Further, the external opticalsystem 392 may comprise one or more amplifiers. In another embodiment,the external optical system 392 comprises an infrared optical parametricoscillator which may be configured to be directly pumped by the outputsignal 380. As such, the external optical parametric oscillator 392 mayinclude one or more non-linear materials such as PPLN, PPLT, PPKTP, KTP,BBO, LBO, and the like. Optionally, the infrared optical parametricoscillator may be cavity dumped to increase the pulse energy and/orintra-cavity frequency doubled to extend the tuning range. The tuningrange of the infrared optical parametric oscillator may be furtherextended to the mid-infrared by difference frequency mixing using thesignal and idler from the OPO, or to the visible by sum frequency mixingusing the signal and pump or idler and pump. Those skilled in the artwill appreciate that any of the laser systems described herein mayinclude one or more external optical systems discussed above.

FIG. 4 shows a schematic of another embodiment of a high-Yb:CaF₂mode-locked laser system. As shown, the laser system 400 includes afirst pump source 402 a and at least a second pump source 402 b. Likethe previous embodiments, the first pump source 402 a and the secondpump source 402 b may comprise a diode-based pump device, although thoseskilled in the art will appreciate that any variety of pump sources maybe used with the laser systems described in the present applications.Further, the first pump source 402 a may be coupled to a first fiberoptic device 406 a. Similarly, the second pump source 402 b may becoupled to a second fiber optic device 406 b. The first pump source 402a is configured to generate at least one pump signal 404 a, while thesecond pump source 402 b is configured to generate at least a secondpump signal 404 b. In one embodiment, at least one of the first andsecond pump signals 404 a, 404 b has a wavelength of about 850 nm toabout 995 nm. For example, in one embodiment, at least one of the firstand second pump signals 404 a, 404 b has a wavelength of about 979 nm.In another embodiment, at least one of the first and second pump signals404 a, 404 b has a wavelength of about 976 nm. In another embodiment, atleast one of the first and second pump signals 404 a, 404 b has awavelength of about 940 nm. In yet another embodiment, at least one ofthe first and second pump signals 404 a, 404 b has a wavelength of about917 nm. Optionally, the first and second pump signals 404 a, 404 b mayhave the same or different wavelengths, polarizations, repetition rates,powers, and the like. Further, at least one of the first and second pumpsources 402 a, 402 b may be configured to output a continuous wave pumpsignal. In another embodiment, at least one of the first and second pumpsources 402 a, 402 b may be configured to output at least one pulsedpump signal.

Referring again to FIG. 4, the first and second pump signals 404 a, 404b may be directed into at least one optical system 410. Those skilled inthe art will appreciate that any number or type of optical components ordevice may be used in the optical system 410. For example, as shown inFIG. 4, at least one of the optical systems 420 comprises beam directorconfigured to transmit substantially all of the second pump signal 404 bthere through while reflecting substantially all of the output signal480. Exemplary other optional optical elements for use in the opticalsystem 410 include, without limitations, lenses, gratings, filters, beamsplitters, sensors, apertures, shutters, modulators, attenuators,homogenizers, polarizers, and the like.

As shown in FIG. 4, the first pump signal 404 a may traverse through atleast one high reflector 420 and may be incident on at least one opticalcrystal system 430 positioned within the laser cavity 490. Similarly,the second pump signal 404 b may traverse through at least one outputcoupler 460 and may be incident on the optical crystal system 430positioned within the laser cavity 490. Like the previous embodiments,the high reflector 420 and output coupler 460 may include one or morecoatings described above applied thereto. Further, the high reflector420 and/or output coupler 460 may comprise planar bodies or curvedbodies.

Referring again to FIG. 4, at least one optical crystal system 430 maybe positioned within the laser cavity 490 and be configured to receiveat least a portion of the first and second pump signals 404 a, 404 btherein. In one embodiment, the optical crystal system 430 comprises atleast one optical crystal 432 positioned on at least one crystal mount434. In one embodiment, the optical crystal 432 comprises at least onebulk, optical material. Exemplary bulk, optical materials include,without limitations, Yb:CaF₂ single crystal materials, Yb:CaF₂ ceramicmaterials, Yb:CALGO, other Yb doped gain media, and the like.Optionally, the Yb:CaF₂ material described herein may include one ormore additional dopants known in the art. For example, in oneembodiment, the Yb:CaF₂ materials described herein may have anabsorption spectrum which extends from about 880 nm to about 1100 nm.Further, the Yb:CaF₂ materials may be configured to generate at leastone intra-cavity signal 436 having a wavelength from about 1000 nm toabout 1700 nm or more in response to being pumped by the first andsecond pump signals 404 a, 404 b. Optionally, multiple optical crystals432 may be positioned within laser cavity 490. Optionally, the crystalmount 434 supporting the optical crystal 432 may be configured to be aircooled, fluid cooled, and the like. As such, the crystal mount 434 maybe in communication with at least one fan, chiller, thermoelectriccooler, sensor, and the like.

Referring again to FIG. 4, the optical crystal 432 is configured togenerate at least one intra-cavity signal 436 in response to beingpumped by at least one of the first and second pump signals 404 a, 404b. The intra-cavity signal 436 may be directed into at least onemode-locking system 440 positioned within or proximate to the lasercavity 490. In one embodiment, the mode-locking system 440 comprises atleast one Kerr lens mode-locking system (hereinafter “KLM system”). Inanother embodiment, the mode-locking system 440 comprises at least onesaturable absorber, semiconductor saturable absorber mirror (hereinafter“SESAM”), and/or hybrid KLM/SESAM system configured to permitself-starting mode-locking. In another embodiment, the mode-lockingsystem 240 comprises a non-linear-optical crystal in which intentionallyphase mis-matched harmonic generation (e.g. second harmonic generation,third harmonic generations, etc.) and provides the self-startingmode-locking. Those skilled in the art will appreciate that any varietyof alternative mode-locking system and devices may be used with thelaser system 400.

As shown in FIG. 4, at least one intra-cavity optical component 450 maybe positioned within the laser cavity 490. In one embodiment, theintra-cavity optical component 450 comprises a spatial filter. Inanother embodiment, the intra-cavity optical component 450 comprises apolarizer. Optionally, any variety of optical components may be used asthe intra-cavity optical component 450, including, without limitations,lenses, beam splitters, mirrors, optical filters, apertures, stops,irises, sensors, prisms, dispersion compensation devices or systems,group delay dispersion devices and systems, Gires-Tournoisinterferometer mirrors, modulators, optical flats, Brewster windows, andthe like. In another embodiment, the intra-cavity optical component 450comprises at least one harmonic generation device or crystal.

Referring again to FIG. 4, the laser system 400 includes at least oneoutput coupler 460, which, in combination with the high reflector 420,defines the laser cavity 490. In one embodiment, the output coupler 460is configured to output an output signal 480 having a wavelength ofabout 1000 nm to about 1700 nm. For example, in one embodiment, theoutput signal 480 has a wavelength between about 1000 nm and 1100 nm.Optionally, the output coupler 460 may include any additional oralternate optical coatings applied thereto. Exemplary additionalcoatings may include, without limitations, polarizing coatings, bandpassfilter coatings, notch filter coatings, wavelength selective coatings,and the like.

FIG. 5 shows a table comparing the performance of the novel bulk Yb:CaF₂laser systems described herein relative to the performance of prior artlaser systems commonly used in multi-photon microscopy applications,optogenetics, micromachining and similar applications. As shown, thenovel bulk Yb:CaF₂ laser systems described herein are capable ofproviding an output signal 280 having an average power in excess ofabout 20 W, with a repetition rate in excess of about 60 MHz, and asub-200 fs pulse width. Optionally, the novel bulk Yb:CaF₂ laser systemsdescribed herein are capable of providing an output signal having anaverage power in excess of about 25 W, with a repetition rate in excessof about 60 MHz, and a sub-200 fs pulse width. In another embodiment,the novel bulk Yb:CaF₂ laser systems described herein are capable ofproviding an output signal 280 having an average power in excess ofabout 30 W or more, with a repetition rate in excess of about 60 MHz,and a sub-200 fs pulse width. Rather, the majority of competing priorart laser system comprise complex thin disk laser systems, which are, bydefinition, not bulk laser systems. Moreover, these disk laser systemsare incapable of providing high power output signals at repetition ratesof greater than about 50 MHz, and sub-200 fs pulse widths. In contrast,as shown in FIG. 5, prior art bulk laser systems are capable ofproviding comparable repetition rates and pulse widths relative to thenovel bulk Yb:CaF₂ laser systems described herein. Unfortunately,however, the prior art bulk laser systems are incapable of provideoutput powers of 20 W or more.

FIG. 6 shows graphically the improved performance achieved by theembodiments of the Yb:CaF₂ mode-locked laser system disclosed herein.More specifically, FIG. 6 shows graphically the range wherein acontinuous wave mode-locked signal is outputted from the output couplerof the laser cavities described herein as a function of output powerversus pump power from the pump source. As discussed above, the cavityof the laser systems described herein may be used with any variety ofoptical crystals, however, these cavities are particularly well suitedfor the inclusion of ytterbium doped optical crystals therein. Allmode-locked oscillators have a limited operating range over which theyproduce a stable train of single ultrafast pulses. The behavior shown inFIG. 1 is representative of oscillators that are mode-locked usingsemiconductor saturable absorber mirrors but other mode-lockingtechniques produce similar results. At low pump power, the oscillatorfirst reaches threshold and then produces cw output with no pulses atall. At higher pump power, a region of q-switched mode-locked (Q-ML)operation is observed until at a yet higher pump power the desired cwmode-locked (CW-ML) performance is achieved. This region of cwmode-locked operation is limited and an instability regime will alwaysoccur at higher pump powers. These instabilities may include, multiplepulsing, spectral instabilities, spatial instabilities and/or temporalinstabilities. As shown in FIG. 6, the CW-ML operational regime of theYb:CaF₂ crystal positioned within the present laser cavities isconsiderably larger than the CW-ML operational regime of prior art lasersystems as shown in FIG. 1. As a result, a higher power, stableintra-cavity signal 236 (See FIG. 2) may be output by the Yb:CaF₂ laserthan prior art systems, thus yielding a higher output signal 280. Thislarge cw mode-locked range is a result of weak thermal lensing, optimumcavity design and optimum saturable absorber design and leads toincreased robustness and manufacturability of the laser system. Thelaser can also be operated with a light loop that modifies the pumppower in order to keep the output power constant without the risk ofoperating in the instability regime.

The embodiments disclosed herein are illustrative of the principles ofthe invention. Other modifications may be employed which are within thescope of the invention. Accordingly, the devices disclosed in thepresent application are not limited to that precisely as shown anddescribed herein.

What is claimed is:
 1. A high-power mode-locked laser system,comprising: at least one pump source configured to output at least onepump signal; at least one laser cavity formed by at least one highreflector and at least one output coupler; at least one Yb-doped opticalcrystal positioned within the at least one laser cavity, the at leastone Yb-doped optical crystal in communication with the at least one pumpsource and pumped by the at least one pump signal, the at least oneYb-doped optical crystal configured to output at least one output signalhaving a output power of 20 W or more and pulse width of 200 fs of less.2. The high-power mode-locked laser system of claim 1 wherein the atleast one pump signal comprises a continuous wave pump signal.
 3. Thehigh-power mode-locked laser system of claim 1 wherein the at least oneoutput signal comprises a continuous wave mode-locked signal.
 4. Thehigh-power mode-locked laser system of claim 1 wherein the at least onepump source comprises multiple laser diode emitters located within asingle diode package coupled to a single fiber optic device, eachemitter configured to output an optical signal into the single fiberoptic device, the single fiber optic device configured to receive theoptical signals from the multiple laser diode emitters and output asingle pump signal.
 5. The high-power mode-locked laser system of claim1 further comprising at least one at least one crystal mount positionedwithin the at least one laser cavity, the at least one crystal mountconfigured to support and position the at least one Yb-doped opticalcrystal at a desired position within the at least one laser cavity. 6.The high-power mode-locked laser system of claim 5 wherein the at leastone crystal mount is manufactured from at least one material having ahigh coefficient of thermal conductivity.
 7. The high-power mode-lockedlaser system of claim 6 wherein the at least one crystal mount includesat least one thermal control feature configured to enhance convectioncooling of the at least one Yb-doped optical crystal positioned on theat least one crystal mount.
 8. The high-power mode-locked laser systemof claim 7 further comprising at least one thermal control systemconfigured to aid in cooling the at least one Yb-doped optical crystal.9. The high-power mode-locked laser system of claim 8 wherein the atleast one thermal control system comprises an air-cooling system. 10.The high-power mode-locked laser system of claim 1 wherein the at leastone Yb-doped optical crystal comprises a <111> cut crystal.
 11. Thehigh-power mode-locked laser system of claim 1 wherein the at least oneoutput signal has a continuous mode-locking range spanning at leastfifty percent (50%) of an upper region of a dynamic range of the outputpower.
 12. A high-power bulk laser system, comprising: at least one pumpsource configured to output at least one pump signal; at least one lasercavity formed from at least one high reflector and at least one outputcoupler; at least one bulk optical crystal positioned within the atleast one laser cavity and in communication with the at least one pumpsource, the at least one bulk optical crystal configured to output atleast one output signal of 20 W or more and 200 fs or less when pumpedwith the at least one pump signal, the at least one output signalconfigured to be outputted from the at least one output coupler.
 13. Thehigh-power bulk laser system of claim 13 wherein the at least one pumpsignal comprises a continuous wave pump signal.
 14. The high-power bulklaser system of claim 13 wherein the at least one output signalcomprises a continuous wave mode-locked signal.
 15. The high-power bulklaser system of the claim 13 wherein the at least one pump sourcecomprises at least one fiber optic device coupled in opticalcommunication with a single diode-based pump source.
 18. The high-powerbulk laser system of claim 13 wherein the at least one pump sourcecomprises multiple laser diode emitters located within a single diodepackage coupled to a single fiber optic device, each emitter configuredto output at least one optical signal into the single fiber opticdevice, the single fiber optic device configured to receive the at leastone optical signal from the multiple laser diode emitters and output asingle pump signal.
 16. The high-power bulk laser system of claim 13wherein the at least one high-power bulk laser system comprises anair-cooled laser system.
 17. The high-power bulk laser system of claim13 further comprising at least one thermal control system configured toaid in cooling the at least one bulk optical crystal.
 18. The high-powerbulk laser system of claim 13 wherein the at least one output signal hasa continuous mode-locking range spanning at least fifty percent (50%) ofan upper region of a dynamic range of the output power.